In-flight jet engine thrust is of interest to atmospheric scientists as well as jet engineers. Accordingly, it is important to develop methods for accurately determining the thrust of a jet engine.
It is well known that both engine power and efficiency are proportional to the product of the thrust times flight velocity. In order to control power and obtain maximum efficiency from, for example, a supersonic jet, thrust can be controlled by variable convergent-divergent nozzles.
In an on-ground jet testing facility, thrust is commonly measured on a thrust stand. While this type of measurement is accurate, the measurement is not truly characteristic of an engine's performance in flight. Furthermore, thrust stand measurements do not give spatially resolved information on the exhaust gases.
Another method for measuring thrust includes adding tracers or particulates to the jet stream to monitor the velocity of the jet stream; this together with information on gas density can yield the thrust. This method is used in some ground-testing facilities, but is unacceptable in many cases. For example, tracers and particulates are often toxic. When used in testing facilities, particulates cause health risks to facility personnel. Furthermore, particles do not follow the flow of the jet stream accurately and therefore, measurements of velocity, and hence thrust are inaccurate.
Exit velocity is often measured using laser-Doppler anemometry (LDA) which requires the presence of particles. LDA, however, does not provide a method for determining gas density. Hence, LDA must be combined with an auxiliary measurement in order to infer mass flux. Therefore, jet engine thrust measurement apparatus typically have several measurement components requiring support systems and data handling systems for each component.
There is an emerging need for non-intrusive measurement of thrust of new hypersonic aircraft engines and mass flux into an engine. There is also a need for a system which utilizes a minimum number of components. Methods utilizing rugged, compact and economical laser sources are also needed.